In Non-Patent Literature 1, a lensless optical correlator using two phase modulation type spatial light modulators (SLMs) is described. The optical correlator described in this literature includes a He—Ne laser light source that outputs laser light, a first reflective SLM that modulates the laser light, a second reflective SLM that is arranged parallel to the first reflective SLM and further modulates light output from the first reflective SLM, and a CCD camera that images light output from the second reflective SLM.